Graph Wall y+ and "y"

k-thie · May 18, 2017, 21:29

It seems very nonsense to ask this one but I am little bit confused.
I am analyzing wall y+ from exporting data (paraview) so I am coding the y+ and u* by matlab.

HVAC room model is 4*4*1.25 m and I take the data at y = 1.245 m from the wall and x = 1,2,3,4 for example.

then from equation to get y+ is y*u_tau/nu.

So I wonder what is y?

What I calculate is 1.25-x??? Which is wrong and I want to use "x" to calculate too.
Could you please enlighten me what is y

piu58 · May 19, 2017, 02:22

y+ is the dimensionless wall distance. If you describe the behavior the velocity near the wall, very different settings lead to similar results if you use y+ instead of y. The velocity has to be normalized too, often it is expressed as a fraction of the free stream velocity, the inlet velocity or an other reference value.

The confusion is, that the y+ tool in openFoam gives the y+ value of the first cell center. But keep in mind, y+ is not a value, but a length measure. It differs from the real y (a coordinate normal to the surface) by a factor which depends on your geometry, and within your geometry, on the place of the wall you want to examine.

If you have the y+ output and the "y value" of the first cell center you may calculate this factor and construct your y+ scale.

I set "y value" in quotes: Please keep in mind that y in the sense of y+ is a coordinate normal to the surface. In most cases it has nothing to do with the y coordinate of your model if you built it cartesian.

LuckyTran · May 19, 2017, 02:41

In the canonical definition of y+, y is the wall-normal distance. But in the canonical setup, there is only one wall and no ambiguity.

In more practical problems with many walls: y is the distance to the nearest wall. This of course leads to interesting questions about what the law of the wall should look in a corner for example when there are two nearby wall.

FMDenaro · May 19, 2017, 04:26

Quote:

Originally Posted by LuckyTran

In the canonical definition of y+, y is the wall-normal distance. But in the canonical setup, there is only one wall and no ambiguity.

In more practical problems with many walls: y is the distance to the nearest wall. This of course leads to interesting questions about what the law of the wall should look in a corner for example when there are two nearby wall.

It simply does not apply...

k-thie · May 19, 2017, 06:31

Theoretically, I understand what is y+ and y.
I would like to plot graph y+ and u+ like this and as you can see that I have a problem with coding numerical result. I am guessing that I would wrongly define y. I just want to know what is y in my case.

LuckyTran · May 19, 2017, 08:54

Quote:

Originally Posted by k-thie

Theoretically, I understand what is y+ and y.
I would like to plot graph y+ and u+ like this and as you can see that I have a problem with coding numerical result. I am guessing that I would wrongly define y. I just want to know what is y in my case.

y should just be the distance to the nearest wall. Since I have no information on what your geometry looks like, I can't give you any hints on how to calculate this y.

k-thie · May 19, 2017, 08:58

Quote:

Originally Posted by LuckyTran

y should just be the distance to the nearest wall. Since I have no information on what your geometry looks like, I can't give you any hints on how to calculate this y.

Here is my geometry.

May 19, 2017, 02:22		#2
piu58 Senior Member Uwe Pilz Join Date: Feb 2017 Location: Leipzig, Germany Posts: 744 Rep Power: 15	y+ is the dimensionless wall distance. If you describe the behavior the velocity near the wall, very different settings lead to similar results if you use y+ instead of y. The velocity has to be normalized too, often it is expressed as a fraction of the free stream velocity, the inlet velocity or an other reference value. The confusion is, that the y+ tool in openFoam gives the y+ value of the first cell center. But keep in mind, y+ is not a value, but a length measure. It differs from the real y (a coordinate normal to the surface) by a factor which depends on your geometry, and within your geometry, on the place of the wall you want to examine. If you have the y+ output and the "y value" of the first cell center you may calculate this factor and construct your y+ scale. I set "y value" in quotes: Please keep in mind that y in the sense of y+ is a coordinate normal to the surface. In most cases it has nothing to do with the y coordinate of your model if you built it cartesian. LuckyTran likes this. __________________ Uwe Pilz -- Die der Hauptbewegung überlagerte Schwankungsbewegung ist in ihren Einzelheiten so hoffnungslos kompliziert, daß ihre theoretische Berechnung aussichtslos erscheint. (Hermann Schlichting, 1950)

May 19, 2017, 02:41		#3
LuckyTran Senior Member Lucky Join Date: Apr 2011 Location: Orlando, FL USA Posts: 5,761 Rep Power: 66	In the canonical definition of y+, y is the wall-normal distance. But in the canonical setup, there is only one wall and no ambiguity. In more practical problems with many walls: y is the distance to the nearest wall. This of course leads to interesting questions about what the law of the wall should look in a corner for example when there are two nearby wall.